A communication speed of a communication system using a digital signal becomes high year by year, and extends from a conventional low speed digital signal having a MHz band to a high speed digital signal having a GHz band in recent years.
Therefore, in the case of carrying out testing of a variety of devices for use in such a communication system, the pulse generator needs to supply a high speed digital signal having a GHz band from a low speed digital signal having a MHz band as a digital signal having applied thereto offset adjustment so as to apply a bias voltage in response to an input interface of equipment targeted to be tested.
As an example, in the case where an output amplitude of a digital signal is from 0.25 Vpp to 2 Vpp (in steps of 2 mV), a bias voltage ranges from −2V to +2V (in steps of 1 mV).
FIG. 7 shows a configuration of a conventional digital signal offset adjusting apparatus 10 used for such a purpose as above described.
This apparatus is generally called a “bias T”, and transmits an alternating current component Dac of a digital signal D input from an input terminal 10a to an output terminal 10b via a capacitor 11.
In addition, one end of a bias applying coil 12 is provided at a terminal at the side of the output terminal 10b of the capacitor 11.
Then, an arbitrary bias voltage Vb is applied from the other end of this bias applying coil 12, whereby a digital signal D′ obtained by superimposing the alternating current component Dac and the bias voltage Vb having passed through the capacitor 11 for signal transmission is output from the output terminal 10b. 
A bias T as described above is described in the following patent documents 1 and 2, for example.
Patent document 1: Jpn. Pat. Appln. KOKAI Publication No. 2004-193275
Patent document 2: Jpn. Pat. Appln. KOKAI Publication No. 2004-193866